TORSIONAL BUCKLING STUDY

Torsional Buckling Study

Torsional Buckling Study- Roof Collapse

Introduction

Despite the great strides forward that have been made in technology, failures continue to occur, often accompanied by great human and economic loss. This text is intended to provide an introduction to the subject of failure analysis. It cannot deal specifically with each and every failure that may be encountered, as new situations are continually arising, but the general methodologies involved in carrying out an analysis are illustrated by a number of case studies.

Background

    In 1970 Vincent Kling agreed to be the architect for the Hartford civic center. Shortly thereafter he hired Fraoli, Blum, and Yesselman, Engineers (F,B,&Y) to design the arena. In order to save money, F,B,&Y proposed an innovative design for the 300 by 360 ft. space frame roof over the arena. The proposed roof consisted of two main layers arranged in 30 by 30-ft grids composed of horizontal steel bars 21-ft apart. 30-ft diagonal bars connected the nodes of the upper and lower layers, and, in turn, were braced by a middle layer of horizontal bars. The 30-ft bars in the top layer were also braced at their midpoint by intermediate diagonal bars.

This design departed from standard space frame roof designing procedures in five ways.

The configuration of the four steel angles did not provide good resistance to buckling.  The cross-shaped built up section has a much smaller radius of gyration than either an I-section or a tube section.

The top horizontal bars intersected at a different point than the diagonal bars rather than at the same point, making the roof especially susceptible to buckling.

The top layer of this roof did not support the roofing panels; the short posts on the nodes of the top layer did. Not only were these posts meant to eliminate bending stresses on the top layer bars, but their varied heights also allowed for positive drainage.

Four pylon legs positioned 45-ft inside of the edges of the roof supported it instead of boundary columns or walls (Levy and Salvadori, 1992).

The space frame was not cambered. Computer analysis predicted a downward deflection of 13-in at the midpoint of the roof and an upward deflection on 6-in at the corners. These deflections were taken into account (ENR, Jan 26, 1978).

Because of these money-saving innovations, the engineers employed state of the art computer analysis to verify the safety of the building.

    A year later construction began. To save time and money, the roof frame was completely assembled on the ground. While it was still on the ground the inspection agency notified the engineers that it had found excessive deflections in some of the nodes. Nothing was done. After the frame was completed, hydraulic jacks located on top of the four pylons slowly lifted it into position. Once the frame was in its final position but before the roof deck was installed, its deflection was measured to be twice that predicted by computer analysis, and the engineers were ...